1. Field of the Invention
The present invention relates to a display device including a pixel array unit where pixel circuits (also referred to as pixels) including an electro-optic element (also referred to as a display element or light-emitting element) are arrayed in a matrix form. More particularly, the present invention relates to an active-matrix display device wherein pixel circuits including an electro-optic element of which the brightness changes depending on the magnitude of a driving signal as a display element are disposed in a matrix form, each pixel circuit has an active element, and display driving is performed in increments of pixel by the active element thereof.
2. Description of the Related Art
As for pixel display elements, there has been a display device employing an electro-optic element of which the brightness changes due to voltage applied thereto or current flowing thereto. For example, as an electro-optic element of which the brightness changes due to voltage applied thereto a liquid crystal display element is a representative example, and as an electro-optic element of which the brightness changes due to current flowing thereto an organic electro luminescence (organic EL, organic light emitting diode (OLED); hereafter, referred to as an organic EL) element is a representative example. An organic EL display device employing an organic EL element which is the latter is a so-called light-emitting display device employing an electro-optic element which is a self-light-emitting element as a pixel display element.
An organic EL element is an electro-optic element employing a phenomenon wherein upon an electric field being applied to an organic thin film, light is emitted. An organic EL element can be driven even with relatively low applied voltage (e.g., at or above 10 V), so can be driven with low consumption power. Also, an organic EL element is a self-light-emitting element which emits light by itself, so with a liquid crystal display device, there is no need to provide an auxiliary lighting member such as a backlight or the like necessary for a liquid crystal device, and accordingly, reduction in weight and reduction in thickness can be readily performed. Further, the response speed of an organic EL element is very high speed (e.g., around several microseconds), so afterimages at the time of moving image display do not occur. According to such advantages, development of flat-self-light-emitting display devices employing an organic EL element as an electro-optic element has been performed in recent years.
With current-driven electro-optic elements including an organic EL element as a representative example, when their driving current values differ, their light-emitting brightness also differs. Accordingly, in order to emit light with stable brightness, it is important to supply a stable driving current to an electro-optic element. For example, a drive system for supplying a driving current to an organic EL element can be divided into a constant current drive system and a constant voltage drive system (no known literature is presented here since this is a well-known technique).
The voltage-current properties of an organic EL element include a property with great inclination, so upon constant voltage driving being performed, minute voltage irregularities or minute element property irregularities cause great brightness irregularities. Thus, in general, constant current driving is employed, which uses a driving transistor at a saturation area. It goes without saying that with constant current driving also, current fluctuation causes brightness irregularities, but small current irregularities cause small brightness irregularities.
Conversely, even with the constant current drive system, in order not to change the light-emitting brightness of an electro-optic element, it is important to steady a driving signal written in and held in a storage-capacitor according to an input image signal. For example, in order not to change the light-emitting brightness of an organic EL element, it is important to steady a driving current according to an input image signal.
Note however, the threshold voltage and mobility of an active element (driving transistor) for driving an electro-optic element fluctuates due to process fluctuation. Also, the property of an electro-optic element such as an organic EL element or the like fluctuates with time. Particularly, in the case of employing a low-temperature-polysilicon TFT substrate or the like, the irregularities of the threshold property and mobility property of a transistor are great. Even with the constant drive system, the property irregularities of such a driving active element or the property fluctuation of an electro-optic element affects light-emitting brightness.
Therefore, in order to control light-emitting brightness across the entire screen of a display device evenly, various types of arrangement have been studied to control light-emitting fluctuation due to the property fluctuation of the above-mentioned driving active element or electro-optic element (see Japanese Unexamined Patent Application Publication No. 2006-215213).
For example, with the arrangement described in Japanese Unexamined Patent Application Publication No. 2006-215213, as a pixel circuit for organic EL elements, there have been proposed a threshold correction function to steady a driving current even in the case of the threshold voltage of a driving transistor having irregularities or change over time, a mobility correction function to steady a driving current even in the case of the mobility of a driving transistor having irregularities or change over time, and a bootstrap function to steady a driving current even in the case of the current-voltage property of an organic EL element having change over time.
In order to realize these threshold correction function and mobility correction function and so forth, it is necessary to turn on/off a sampling transistor or each transistor added for threshold correction or mobility correction at a predetermined timing using a pulse signal.
The ON period or OFF period of each transistor determines each correction period, so it is important to manage timing for turning on/off each transistor to receive each correction effect. Upon irregularities being caused with the correction period, the threshold correction advantage and mobility correction advantage fluctuate from one pixel to another, and brightness unevenness due to such irregularities is caused, leading to image quality deterioration. For example, there is no problem in the case of the irregularities between the correction periods with leeway, such that when the correction period is long, even if there are a few irregularities regarding ON/OFF timing, there are few problems, but the shorter the correction period becomes, the smaller the leeway as to the irregularities between the correction periods becomes, and accordingly, it is important to manage so as not to cause irregularities thereof, and so as not to cause deviation regarding the ON/OFF timing of a transistor.
Now, a pulse signal (pulse signal for brightness change correction operation) for controlling each transistor is output for each scanning line from a scan circuit provided on the side edge of a pixel array unit where pixel circuits are arrayed in a two-dimensional form, and is supplied simultaneously for each scanning line to predetermined terminals of all the pixel circuits connected to each scanning line within the pixel array unit via each scanning line. Note however, a scanning line having linear resistance and distributed capacity (overlap parasitic capacitance), so there is concern that there may be difference regarding the propagation property of a pulse signal depending on whether the pixel circuit is far from or near the scan circuit, and the correction period fluctuate due to the propagation property difference thereof. Focusing attention on this point, it can be conceived to employ a technique for improving the irregularities between the correction periods from the perspective of driving timing.